Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with wireless communication units within a relatively large geographical coverage area. Typically, wireless communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called Iub interface.
The second generation wireless communication system (2G), also known as GSM, is a well-established cellular, wireless communications technology whereby “base transceiver stations” (equivalent to the Node B's of the 3G system) and “mobile stations” (user equipment) can transmit and receive voice and packet data. Several base transceiver stations are controlled by a Base Station Controller (BSC), equivalent to the RNC of 3G systems.
Communications systems and networks are developing towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network, and a System Architecture Evolution (SAE) solution, namely, an Evolved Packet Core (EPC), for a mobile core network. An evolved packet system (EPS) network provides only packet switching (PS) domain data access so voice services are provided by a 2G or 3G Radio Access Network (RAN) and circuit switched (CS) domain network. User Equipment (UE) can access a CS domain core network through a 2G/3GRAN such as the (Enhanced Data Rate for GSM Evolution, EDGE) Radio Access Network (GERAN) or a Universal Mobile Telecommunication System Terrestrial Radio Access Network (UTRAN), and access the EPC through the E-UTRAN.
Some user equipments have the capability to communicate with networks of differing radio access technologies. For example, a user equipment may be capable of operating within a UTRAN and within an E-UTRAN.
Lower power (and therefore smaller coverage area) cells are a recent development within the field of wireless cellular communication systems. Such small cells are effectively communication coverage areas supported by low power base stations. The terms “picocell” and “femtocell” are often used to mean a cell with a small coverage area, with the term femtocell being more commonly used with reference to residential small cells. Small cells are often deployed with minimum RF (radio frequency) planning and those operating in consumers' homes are often installed in an ad hoc fashion. The low power base stations which support small cells are referred to as Access Points (AP's) with the term Home Node B (HNB's) or Evolved Node Node B (eHNB) identifying femtocell Access Points. Each small-cell is supported by a single Access Point. These small cells are intended to augment the wide area macro network and support communications to multiple User Equipment devices in a restricted, for example, indoor environment. An additional benefit of small cells is that they can offload traffic from the macro network, thereby freeing up valuable macro network resources An HNB is an Access Point that provides a wireless interface for user equipment connectivity. It provides a radio access network connectivity to a user equipment (UE) using the so-called Iuh interface to a network Access Controller, also known as a Home Node B Gateway (HNB-GW). One Access Controller (AC) can provide network connectivity of several HNB's to a core network.
Typical applications for such Access Points include, by way of example, residential and commercial locations, communication ‘hotspots’, etc., whereby Access Points can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, small cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, network congestion or poor coverage at the macro-cell level may be problematic.
In some applications, small Access Points (or Home Node B's are provided with a network listen device or module which listens to broadcasts from base stations (eg. Access Points, Node B's and Home Node B's) serving neighbouring cells.
Thus, an AP is a scalable, multi-channel, two-way communication device that may be provided within, say, residential and commercial (e.g. office) locations, ‘hotspots’ etc, to extend or improve upon network coverage within those locations. Although there are no standard criteria for the functional components of an AP, an example of a typical AP for use within a 3GPP 3G system may comprise Node-B functionality and some aspects of Radio Network Controller (RNC) functionality as specified in 3GPP TS 25.467. These small cells are intended to be able to be deployed alongside the more widely used macro-cellular network and support communications to UEs in a restricted, for example ‘in-building’, environment.
Herein, the term “small cell” means any cell having a small coverage area and includes “picocells” and “femtocells.”
In a planned cellular network, a so-called neighbour cell list is used to identify adjacent cells to each cell, to facilitate handover of UE communications from a “source” cell to a “target” cell when the strength or quality of the signal from the serving (source) cell, for example, becomes too poor to maintain the communication. The neighbour cell list is broadcast to UEs to enable the UE to receive and assess the suitability of continuing a communication by transferring the communication to an adjacent (neighbour) cell. A neighbour cell list contains, inter alia, cell ID, frequency and scrambling code information for all of the cells whose coverage area overlaps with the UE's current serving cell, to allow the UE to be able to receive and decode transmissions from the neighbouring cells and send back measurement reports. A UE can use, for example, the conventional intra-frequency PSC detection mode to detect neighbouring cells operating with the same ARFCN (Absolute Radio Frequency Channel Number) and send measurement reports back to its serving node B or AP. Such measurement reports can include signal levels and signal quality of the transmissions from a detected cell. An Access Point may be provisioned with a neighbour cell list prior to deployment In other arrangements, an AP may not be provisioned with a neighbour cell list but instead, it may configure the list itself based on measurements of signals (broadcast by neighbouring cells) detected by its network listen module.
For an AP that relies on its network listen module to self-configure a neighbouring cell list, a problem arises when the network listen scan does not result in finding any neighbouring cells, either intra-frequency (co-channel) or inter-frequency. In such cases, the risk of a UE close to the cell edge dropping a call is increased since no handover target cell has been identified. A similar problem can arise in cases where the network listen module has found only intra-frequency neighbouring cells but the system Operator has disabled intra-frequency handover for that particular AP. Even if the network listen module does find one or more neighbouring cells, the any neighbour cell list that the AP may subsequently compile will be incomplete if there are other neighbouring cells which are “visible” to the UE but not to the network listen module. Having a limited number of configured neighbouring cells in a neighbour cell list can have a deleterious effect on the stability of any handover process.